1. Field of the Invention
The present invention relates to a semiconductor device having a semiconductor circuit that comprises a plurality of semiconductor units such as insulated gate transistors and the like, especially to that having a semiconductor circuit that comprises an active matrix unit (high withstand voltage circuit) and a logic circuit unit (high-speed driving circuit) for driving the active matrix unit both formed on one substrate, and also relates to a method for fabricating the semiconductor device. The semiconductor circuit to be constructed according to the invention may be on any insulation substrates of, for example, glass or the like, or on any insulation films as formed on semiconductor substrates of, for example, single-crystal silicon or the like.
In particular, the invention is especially favorable to and effective for semiconductor devices comprising a pixel matrix portion and a logic circuit for driving it, such as, liquid-crystal display devices, etc. The semiconductor device of the invention includes not only insulated gate transistors (semiconductor units) such as thin film transistors (TFTS), MOS transistors, etc., but also display devices having a semiconductor integrated circuit that comprises such semiconductor units, even further including electro-optical devices such as image sensors, etc. In addition, the semiconductor device of the invention still further includes electronic appliances incorporating any of those display devices and electro-optical devices.
2. Description of the Related Art
Active matrix-type liquid-crystal display devices are widely noticed in the art, which comprise a pixel matrix and a driving circuit comprising thin film transistors (TFTS) and the like as formed on an insulation substrate. Liquid-crystal panels having a size of from 0.5 to 20 inches or so are utilized as image-displaying panels.
One direction of liquid-crystalline display development is toward large-area display panels. In display devices having a large-area display panel, the pixel matrix to be the image-displaying portion shall have a large area and the source interconnections and also the gate interconnections as aligned in matrices in the circuit shall be long with the result that the circuit inevitably shall have increased interconnection resistance. In addition, the circuit must be patterned finer, for which the interconnection patterns shall be inevitably finer. Such finer interconnection patterns increase more the interconnection resistance. In each pixel, the source interconnection and the gate interconnection are connected with individual switching units such as TFTs and others. With the increase in the number of pixels, the incidental capacity increases, which, however, is problematic. In liquid-crystal display devices, in general, the gate interconnection pattern is integrated with the gate electrode pattern. In those, therefore, increasing the panel area often causes substantial gate signal delay.
If the materials constituting a gate interconnection have a lower resistivity, it is possible to modify the gate interconnection patterns to be finer and longer; therefore, enable large-area display panels. Al, Ta, Ti and the like have heretofore been used as the materials constituting the gate interconnection. Of those, Al is the most popular, as having a lowest resistivity and capable of being subjected to anodic oxidation. An oxide film as formed through anodic oxidation of Al has the advantage of improving the heat resistance of Al patterns coated with the oxide film. However, Al patterns are still problematic in that, even at a process temperature falling between 300° C. and 400° C. or so, they give whiskers and hillocks and are often deformed. In addition, Al readily diffuses into insulation films and active layers, thereby causing TFT operation failure, and TFT characteristics are much degraded by it.
For further enlarging the panel size of display devices and for ensuring finer patterning in fabricating them, needed are electrode structures having a much lower resistivity and better heat resistance.
Another direction of liquid-crystal display development is toward large scale integration of semiconductor units. For this, generally known is a peripheral driving circuit-integrated structure, for which a pixel matrix and a peripheral driving circuit are integrated and mounted on one and the same substrate. The peripheral driving circuit-integrated structure of that type has the advantages of low production cost and compact size.
As a rule, in an ordinary pixel matrix, either one of p-channel or n-channel thin film transistor (TFT) is disposed as a switching unit. A peripheral driving circuit for driving the pixel matrix comprises a logic circuit unit (high-speed driving circuit). One typical example of the peripheral driving circuit comprises a CMOS circuit unit composed of P-channel and N-channel TFTS.
For example, the TFTs disposed in the pixel matrix portion is required that each of the TFTs has a lower OFF current characteristic, because the pixel electrodes disposed in the pixel matrix portion is required to have a charge-retaining function.
On the other hand, the TFTs constituting the logic circuit disposed to be the peripheral driving circuit is required to operate quickly even at low current.
As in the above, the pixel matrix and the logic circuit differ from each other with respect to the necessary TFT characteristics for them. Therefore, it is desirable to prepare different TFT structures for the pixel matrix and the logic circuit and to fabricate the pixel matrix and the logic circuit by the use of those different TFT structures as separately prepared for them. However, the process of separately fabricating the pixel matrix and the logic circuit and integrating them is complex, and the yield in the process is low, and, in addition, the production costs are high. The complicated process to lower the yield and to increase the production costs is undesirable.
Given that situation, it is desired to form TFTs for the pixel matrix and those for the logical circuit all on one and the same substrate in a continuous process for fabricating peripheral driving circuit-integrated liquid-crystal display devices.
As one means for solving the problems noted above, known is a thin film transistor structure having an LDD (lightly doped drain) region. The LDD region is to relax the field strength to be formed between a channel forming region and a drain region, while lowering the OFF current in thin film transistors and preventing the thin film transistors from deteriorating. Having such an LDD region, thin film transistors ensure a lower OFF current characteristic.
In conventional techniques, the LDD region is formed from an anodic oxide film in a self-aligned process. However, the self-aligned process is not suitable for forming fine patterns, in which the patterned line width and the condition for anodic oxidation are difficult to control.